1. Field of the Invention
The present invention relates to a process for recovering a processing liquid and, more particularly, to a process which minimizes energy requirements and capital expenditures.
2. Description of the Prior Art
There are numerous industrial processes wherein a liquid, hereinafter referred to as a processing liquid, which can comprise one or more components, is used in such a fashion that it becomes contaminated with, or contains, various components, some of which are more volatile than the processing liquid and some of which are less volatile and can be dissolved in the processing liquid. Usually, the components in the processing liquid are contaminants, although they may be desirable recovered components, depending on the process in which the processing liquid is used. In such cases, it is almost universally desirable to separate the processing liquid from the less volatile and more volatile components so that the processing liquid can be reused in the process or simply recovered in a substantially pure state for reuse or other uses.
Numerous examples of the above described general scheme of using a processing liquid abound. For example, it is well known that natural gas produced from oil and gas wells, in addition to containing gaseous hydrocarbons, such as methane, ethane, etc., almost invariably contains water and acidic gases, such as CO2 and H2S. In cases where the natural gas contains water, it is very common for so-called gas hydrates or clathrate hydrates to form. These clathrate hydrates are crystalline compounds that occur when water forms a cage-like structure around guest molecules, particularly gaseous molecules.
While the phenomena can occur in any system wherein there is water and gaseous compounds, e.g., hydrocarbons, the problem, at times, becomes especially acute in the petroleum industry, not only with respect to the production of gaseous hydrocarbons such as natural gas, but also in the transporting and processing of natural gas. As noted, typical gas hydrates formed in petroleum (hydrocarbon) environments are composed of water and one or more guest molecules, such as methane, ethane, propane, isobutane, nitrogen, carbon dioxide, and hydrogen sulfide. However, it is also known that other guest molecules such as nitrous oxide, acetylene, vinyl chloride, ethyl bromide, oxygen, etc., can form clathrate hydrates.
With particular reference to natural gas systems and by example only, when gas hydrate crystals form, they can become a nuisance at least and pose a serious problem at worst. Gas hydrates can block transmission lines and plug blowout preventers, jeopardize the foundations of deep water platforms and pipelines, collapse tubing and casing, and foul process equipment, such as heat exchangers, compressors, separators, and expanders. To overcome these problems, several approaches are possible in principal: removal of free water, maintaining an elevated temperature and/or reduced pressure, or the addition of freezing point depressants. As a practical matter, the last mentioned measure, i.e., adding freezing point depressants, has been most frequently applied. Thus, lower alcohols, such as methanol, ethanol, etc., and glycols have been added to act as antifreezes.
While processing liquids such as alcohols and glycols used in natural gas production, transportation, and processing are effective at reducing gas hydrate formation, their use is not without problems. As is well known, the production of natural gas is frequently accompanied by the production of brine, containing sodium chloride and other water-soluble salts. While these halides, such as the alkali metal halides, are readily soluble in water, they also exhibit substantial solubility in the alcohols and glycols used to prevent gas hydrate formation. Accordingly, the processing liquid—in this case the alcohol, glycol, or the like—becomes contaminated with dissolved salts present in the produced water, as well as with certain gases, which, depending on the particular gas, are soluble in the processing liquid. Thus, this presents a specific example where a processing liquid has been used, in this case to prevent hydrate formation, and has now become contaminated with a more volatile component and a less volatile, and in this case dissolved, component.
Again, using the example of natural gas production, transportation, and processing, it is necessary that the natural gas be freed of acidic components, such as CO2, H2S, sulfur oxides, etc., some of which are quite toxic, all of which can lead to severe corrosion problems and in certain cases the formation of unwanted by-products. It is common to scrub the natural gas stream with processing liquids such as liquid amines, particularly alkanolamines such as monoethanolamine (MEA); diethanolamine (DEA); methyldiethanolamine (MDEA), proprietary blends of additives and alkanolamines, as well as glycols such as mono-, di-, or tri-ethylene glycol and non-aqueous heat transfer fluids. Since scrubbing of natural gas to remove acidic gases is normally conducted on natural gas streams that have been substantially freed of water, the dissolved salt content of the natural gas stream from the gas stream is generally quite small. However, even though the ingress of dissolved salt is low from the natural gas stream, continuous use of the amine process liquid for acid gas removal tends to cause the amine to break down with contaminants and create heat-stable, unregenerable salts. If the residual buildup of heat-stable salts (HSS) is permitted to build to typical levels in excess of 1% by weight, the amine performance will decline, corrosion increases rapidly with a decline in pH, and the amine solution begins to foam, creating excessive process liquid losses. Accordingly, the processing liquid, e.g., the alkanolamine, will generally contain dissolved, less volatile components at a much smaller concentration than in the case of an alcohol or glycol used to prevent gas hydrate formation. Nonetheless, even in this instance, the processing liquid now presents a case where, after use, it contains more volatile components, e.g., CO2, H2S, etc., and perhaps a small amount of less volatile and dissolved component.
In the case where treatment of the natural gas to prevent gas hydrate formation and/or remove acidic gases is conducted on offshore platforms, several problems are encountered. For one, the alcohols, glycols, and alkanolamines can be toxic to marine life and accordingly, once spent, e.g., saturated with contaminants that they are being used to remove, cannot be discharged overboard. Aside from ecological concerns, such a method is economically not feasible since it requires a constant replenishment of the processing liquid. Indeed, such a process would not be economically feasible in land-based refineries, chemical plants, or the like.
An evolving area for using alkanolamines as well as proprietary blends of alkanolamines, amines and additives is in greenhouse gas emission abatement from utility vent and flue gas streams—collectively called exhaust gas streams—associated with refineries, gas plants as well as thermal energy production facilities. Depending on the type of fuel used, for example, natural gas, residue coke, bunker oil, heavy oil fractions, all grades of coal, etc., the resulting exhaust gas stream can contain a wide variety of contaminants. These contaminants may include acid gases and solids that differ from those found in natural gas; by way of example, sulphur oxides, nitrogen oxides, carbon dioxide, particulate matter and others that can be created when the aforementioned contaminants react with the processing liquid to degrade the processing liquid into organic byproducts that have reduced gas treating capacity and that may also contribute to processing solution corrosivity. The contact between the exhaust gas stream and the processing liquid results in these contaminants being removed from the exhaust gas stream along with the target greenhouse gases, such as carbon dioxide. Over a relatively short period of time the buildup of these contaminants results in a decline in greenhouse gas removal efficiency and would render the processing liquid unusable if these contaminants were not removed or at least reduced sufficiently as to not interfere with the ability of the processing liquid to capture greenhouse gases within the exhaust gas stream. Plant metallurgy would also be in jeopardy if the corrosivity of the processing liquid is not kept relatively benign. To maintain optimal removal of these greenhouse gases and prevent accelerated corrosion, the processing liquid must have these contaminants removed on a regular basis and ideally on a continual basis.
A secondary artifact of the processes being used for greenhouse gas removal is that, in addition to the contaminants being captured along with the greenhouse gases, the cyclic process being used only partially removes the greenhouse gases from the regenerated processing liquid prior to it being cooled and sent back to the front of the process to remove more greenhouse gases and by their very presence, more of the associated contaminants. Any contaminant removal method considered has to deal with this high concentration of greenhouse gases remaining in the process solvent requiring reclaiming.
U.S. Pat. Nos. 5,152,887; 5,158,649; 5,389,208; and 5,441,605, all of which are incorporated herein by reference for all purposes, all deal with processes and apparatus for reclaiming and/or concentrating waste aqueous solutions of gas treating chemicals. Additionally, U.S. Pat. Nos. 4,315,815, and 4,770,747, both of which are herein incorporated by reference for all purposes, likewise deal with processes for reclaiming or recovering gas-treating liquids. U.S. Pat. No. 5,389,208 discloses and claims a method for reclaiming an impurity-containing waste aqueous solution of a gas-treating chemical that basically involves vacuum distillation of the spent material under temperature conditions that prevent decomposition of the gas-treating chemical and in such a fashion that the process can be operated in apparatuses made of carbon steel, as opposed to more exotic materials of construction, without causing substantial corrosion of the apparatus.
In U.S. Pat. No. 5,993,608 and U.S. Pat. No. 6,508,916, both incorporated herein by reference for all purposes, disclose and claim processes for recovering processing liquids wherein components less volatile than the processing liquid such as dissolved and/or suspended solids are removed from the processing liquid under conditions that prevent any substantial degradation of the processing liquid and provide for the recycle water, refined processing liquid or a mixture thereof back to the front end of the process.
It will be appreciated that the processing liquid is generally expensive and furthermore cannot generally be disposed of in an environmentally suitable manner. Accordingly, the goal of all processes to recover or clean used processing liquids is to render the cleaned processing liquid suitable for further use in the process from which it came. In many processes, the cleaned processing liquid is sent to storage or surge tanks from which it is removed for reuse in a gas processing facility. These processes do not make optimal use of available energy required for cleaning used processing fluid.